The present invention generally relates to seat belt retractors having energy-absorbing or dissipating mechanisms and more particularly to a multi-level energy absorbing or dissipating system, which is also referred to as a multi-level load limiting system. The system includes a device for shifting the output characteristic of the retractor from one load level limit (energy dissipation) to another load level limit (energy dissipation) based on operational parameters.
Torsion bars have for decades been proposed for use in seat belt retractors. As the torsion bar is twisted during a vehicle emergency, the retractor output, that is the force displacement (force rotation) characteristic of the torsion bar quickly reaches a saturated region, which corresponds to its plastic range of operation. This generally constant characteristic provides a reaction force or load on the seat belt that may not be adequate to protect occupants of differing sizes. Consequently, it is desirable to provide a seat belt system with more than one load-limiting characteristic. As the torsion bar is twisted crash energy is absorbed (dissipated).
The prior art illustrates seat belt retractors having two dissimilar and remotely located torsion bars to achieve a multi-level of load limiting, while other prior retractors use a single torsion bar that is sub-divided into two torsion bar portions to achieve multi-level load limiting operation.
The present invention utilizes, in the illustrated embodiments, a single torsion bar to achieve a first level of load limiting under certain operating conditions and includes a mechanism to disable the torsion bar during other operating conditions. To achieve the desired second level of load limiting a variable elongation seat belt webbing is used. One such webbing is shown in U.S. Pat. Nos. 5,830.811 and 6.228,488, which are incorporated herein by reference.
A typical woven seat belt displays a generally determinable elongation rate in the range of about 8-12 percent. For any seat belt the elongation does not change appreciably when it is loaded and operating in its elastic mode. The variable elongation seat belt referred to above has an elongation rate that varies with the applied load. For example, at applied belt forces or loads of 4-5 kilo-Newtons the elongation rate is approximately 6-8 percent. As the load increases, the seat belt becomes more slack and its elongation rate increases to approximately 12-14 percent. The increased elongation rate, that is, the increased stretchiness of the belt permits the occupant to move forward during a crash, as crash energy is absorbed by the belt. As the seat belt stretches crash energy is dissipated.
The classic seat belt retractor only offers a modest degree of energy absorption, which occurs as the seat belt stretches as it is loaded by the occupant during an emergency.
A newer class of seat belt systems has incorporated various types energy-absorbing mechanisms including crush bushings and torsion bars, which permit the seat belt spool and hence the webbing (wound thereabout) to protract in a controlled manner. In this type of system, the applied force generated by the occupant is resisted by a controlled force created as the torsion bar twists. As the torsion bar twists, crash energy is absorbed or dissipated; additionally the controlled amount of the seat belt protracted from the spool lessens the likelihood of occupant injury.
It is an object of the present invention to provide a multi-level energy-absorbing seat belt retractor.
The present invention advantageously utilizes the combination of the characteristics of the torsion bar and the variable elongation webbing to provide a multi-level seat belt load limiting system. In other embodiments the energy-absorbing (energy-dissipating) characteristics of the torsion bar are replaced by a friction mechanism while still using the variable elongation webbing.
Many other objects and purposes of the invention will be clear from the following detailed description of the drawings.